Analogue to digital conversion by glassware!

This is an extract from an article on pulse code modulation, from Electronic
Engineering magazine, April 1953, describing a very ingenuious use of a cathode-ray tube
to perform 7-bit analogue to digital conversion, outputting a serial binary data stream
from an analogue input.

Meacham and Peterson* have described a system which uses a special form of
cathode-ray tube for the coder. The signal of each channel is sampled every 125
microseconds and the outputs from 12 channel modulators are commoned to form a
pulse-amplitude modulated T.D.M. system. Because the operation of the coder takes longer
than 10.4 microseconds two coders are provided, each dealing with alternate channels.

The essential features of the coding circuit are shown in Fig. 38 and photographs of
the coding tubes' and its aperture plate are shown in Fig. 39. The aperture plate is
perforated with the 128 combinations of 7 binary digits with holes for units and solid
metal for zeros. When-the electron beam is switched on it is deflected by the stored
sample voltage to the corresponding vertical height and is moved across the aperture plate
by a linear sweep. As the beam passes the holes in the aperture plate current flows to the
collector plate producing the coded group of output pulses; the beam is switched off and
retraced in readiness for coding the next sample. In front of the aperture plate is the
quantizing grid of 129 wires between which are spaces in line with the row of holes in the
aperture plate. The wires are used to centre the beam accurately on a row of holes in the
aperture plate: without it the beam might wander between two rows during the scan and
false coding would result. During the scan a bias voltage is applied which tends to move
the beam upward, but when it strikes one of the wires of the quantizing grid secondary
electrons are emitted which are attractzd to the collector. The signal from the collector
is fed back to the vertical amplifier and deflects the beam downwards so that it is kept
in a stable position just beneath the quantizing wire.

The aperture of the coding tube viewed from the gun end; (below) the
complete tube

Thanks to Joe Sousa for the following info : I was particularly impressed by
the 8 bit glass ADC from 1948, because I have been designing ADC's for 20 years. You
should know that in the 1980's Tektronix sold a 7912AD digital storage oscilloscope that
used a similar glass CRT to digitize single-shot waveforms up to 500MHz. This
link will show a picture of the scope. This scope had GPIB digital output or
NTSC video output, where the 512x512 diode array inside the CRT face was read at NTSC
video rates. Just imagine how wonderfully geeky it would be to hook up this scope to a
32"(81cm) home TV. I have never seen the actual ADC tube.

Below is a device along similar lines, from Wireless World, March 1964

A variety of applications are envisaged for the Raytheon CK1378 cathode-ray tube. The
electron beam produced by a normal gun is deflected into one of thirty-six sectors on the
"screen" of the tube, each sector being electrically isolated from the rest.
Output leads from the sectors are taken to four E9 nine-pin bases mounted on the tube
face, which is also made fluorescent in the normal way, so that the position of the beam
can be seen. The tube can be used for multiplexing, frequency multiplication,
analogue-to-digital conversion and many other applications.

And while we're on
the subject of wacky CRT's.... how about this - a tube with EIGHT independent electron
guns! Made by 20th Centuary Electronics, from Wireless World, June 1957.